Triconstituent Coassembly to Mesoporous Carbon and Its Application

Article pubs.acs.org/jced

Triconstituent Coassembly to Mesoporous Carbon and Its Application to the Enhancement of CO2 Storage in the Presence of Water Wei Su,† Meng Ran,† Jia Zhou,† Yan Sun,‡ Jia Liu,§ and Xiaojing Wang*,† †

Tianjin Key Laboratory of Membrane and Desalination Technology, School of Chemical Engineering and Technology and Department of Chemistry, School of Science, Tianjin University, Tianjin 300350, P.R. China § Nanyang Technological University School of Physics and Mathematics CBC, Singapore ‡

ABSTRACT: Mesoporous carbon materials were synthesized via the triconstituent Co-assembly of resol, tetraethylorthosilicate (TEOS), and pluronic F127, followed by the process of polymerization. The synthesized mesoporous carbons exhibit uniform large pore sizes (3.6−6.2 nm), high surface area (940−1310 m2/g), and large pore volume (1.16−1.71 m3/g). Adsorption isotherms of CO2 on sample MC2 were measured at 275 K with various amounts of water preadsorbed. The highest CO2 sorption capacity reaches about 51.8 mmol/g when the weight ratio of water to dry carbon (Rw) is 2.55, which is 1.23 times as high as the highest CO2 sorption previously obtained on CMK-3 in the presence of water. A storage capacity of 269 (V/V) was obtained at 275 K under 3.5 MPa with a packing density of 0.30 cm3/g. By adding tetrahydrofuran (THF) in water as a hydrate promoter, the hydrate formation pressure was decreased from 1.75 to 0.25 MPa with a THF concentration of 6.7 (mol %).

1. INTRODUCTION

It is difficult to control the pore size of conventional porous carbon such as activated carbon when prepared by the chemical or physical activation of precursors.20 However, it is easy to control and adjust the pore size for porous carbons synthesized via a template.21 Compared to the hard template method, the synthesis process via a soft template is simpler and cheaper. In this study, mesoporous carbons were synthesized via the triconstituent coassembly of resol, tetraethylorthosilicate (TEOS), and pluronic F127, followed by polymerization. CO2 sorption isotherms were tested in the presence of water. In addition, the CO2 storage capacity and sorption mechanism were investigated.

CCUS (carbon capture, utilization, and storage) technology has received more and more attention with global warming. Gas hydrate technology will be one CCUS option, which is a promising alternative to capturing CO2 and reducing carbon emission for its advantages of large adsorption capacity, small investment, and low cost.1 However, CO2 hydrates form very slowly in bulk pure water. The kinetics can be enhanced by adding hydrate promoters such as cyclopentane,2 propane,3 tetra-n-butylammonium bromide,4 sodium dodecyl sulfate,5 tetrahydrofuran (THF),6 and so on, and the hydrate formation pressure decreases considerably as well. CO2 hydrate formation is accelerated in porous media because of its large contact area between gas and liquid.7 A wide variety of porous materials have been reported to form CO2 hydrates, such as silica gel,8,9 glass beads,10 KIT-6,11 MIL-53,12 CMK-3,13 activated carbon,14,15 and so on. Kim et al.12 investigated the formation of CO2 hydrate in metal organic framework material MIL-53 and found that the CO2 hydrate cannot form in micropores but only in mesopores. The total pore volume of KIT-6 is about 1.43 cm3/g, which is higher than that of CMK-3. However, the sorption capacity of CO2 for CMK-3 in the presence of water is about 42 mmol/g at 275 K under 3.6 MPa, which is much higher than that for KIT-6 and other silica materials.11 Similar results have been reported for methane storage in the presence of water.16−19 It is pointed out that porous carbon is more suitable to be employed as a medium to form gas hydrates. © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals and gases used in this work are presented in Table 1. 2.2. Synthesis. First, the procedure for resol synthesis22 was as follows: phenol (0.61 g) was melted in a flask at 313−315 K, and then a NaOH aqueous solution (0.13 g, 20 wt %) was added with stirring. After stirring for 10 min, formaldehyde (1.05 g, 37 wt %) was added to the solution dropwise, and the mixture was stirried for 1 h at 343 K. After the mixture was cooled to room temperature, the solution was adjusted to neutral pH with HCl solution (2 mol/L). The water in the Received: December 12, 2016 Accepted: February 23, 2017

A

DOI: 10.1021/acs.jced.6b01030 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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3. RESULTS AND DISCUSSION 3.1. Synthesis of Mesoporous Carbons. To investigate the effects of a reagent dosage, a series of mesoporous carbons named MCn were prepared with different proportions of reagents. The reagent dosage of different MCs and their textural properties are listed in Table 2. The N2 adsorption−

Table 1. Chemicals and Gases Used name pluronic F127 tetraethylorthosilicate (TEOS) hydrochloric acid hydrofluoric acid phenol sodium hydroxide tetrahydrofuran (THF) ethanol formaldehyde helium nitrogen carbon dioxide

source

purity

Sigma-Aldrich (Shanghai) Trading Co., Ltd. Tianjin Jiangtian Co., Ltd.

Mw = 12 600

Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Jiangtian Co., Ltd. Tianjin Liufang Industrial Gases Co., Ltd. Tianjin Liufang Industrial Gases Co., Ltd. Tianjin Liufang Industrial Gases Co., Ltd.

≥99.7% ≥99.7% ≥99.7% ≥99.7% ≥99.7% ≥99.9% 37 wt % ≥99.9995%

≥99.7%

Table 2. Textural Properties of the Mesoporous Carbon Materials

≥99.9995% ≥99.9995%

samples

F127 (g)

resol (g)

HCl (mol/L)

TEOS (g)

the specific area (m2/g)

MC1 MC2 MC3 MC4 MC5 MC6 MC7

0.50 1.00 2.00 1.00 1.00 1.00 1.00

2.50 2.50 2.50 1.50 3.50 2.50 2.50

0.20 0.20 0.20 0.20 0.20 0.50 1.00

2.08 2.08 2.08 2.08 2.08 2.08 2.08

1030 1300 1308 1289 947 1112 1126

pore volum (cm3/g)

pore size (nm)

1.32 1.61 1.43 1.16 1.45 1.71 1.63

5.1 5.0 4.4 3.6 6.1 6.2 5.8

desorption isotherms on these MCs at 77 K and their pore size distributions are presented in Figure 1. The isotherms are all type IV with hysteresis loops, indicating that the materials are mesoporous. As shown in Table 2, the mass ratio of reactants and concentrations has an important effect on the textural properties of the mesoporous carbon materials. When the mass ratio of reactants and concentrations is F127/resol/HCl/ TEOS = 1.00:2.50:0.20:2.08, the obtained mesoporous carbon (MC2) has a relatively larger specific surface area (1300 m2/g), larger pore volume (1.61 cm3/g), and a pore size centered around 5 nm, which are favorable for the CO2 storage. Therefore, the CO2 sorption was tested on this mesoporous carbon. 3.2. CO2 Sorption. The CO2 sorption isotherms on the mesoporous carbon (MC2) with different Rw at 275 K are shown in Figure 2. Rw varied from 0 to 2.95. The CO2 sorption capacity is expressed in mmol CO2 per gram of dry mesoporous carbon sample. The CO2 sorption isotherm on the dry mesoporous carbon sample (curve 1, Rw = 0) is type IV because its mesoporosity and CO2 sorption capacity are about 28 mmol/g at 3.6 MPa. After loading water into the dry carbon, Rw varied from 2.05 to 2.55 (curve 2−4), and a sharp increase in sorption appeared at about 1.75 MPa as a result of CO2 hydrate formation.8 The inflection pressure and CO2 hydrate formation pressure in the porous materials are higher than the CO2 hydrate formation pressure in bulk pure water (1.5 MPa)13 Higher pressures are needed to form gas hydrates in porous materials.25 Similar results have been reported in previous work.11,13,15 When the pressure reached about 2.5 MPa, another sharp increase appeared. When the water content was higher than the total pore volume, water occupied both the inner and external spaces of the pores. The inner space of the pores consisted of the pore measured by N2 adsorption at 77 K, and the external space of the pores consisted of the interparticle space and macropore of carbon. Hydrate formation in the inner pores requires higher pressure.8 While Rw reached 2.95 (curve 5), the sorption capacity decreased rapidly. This is because too much water makes it difficult to form gas hydrates.11 The highest sorption capacity is up to 51.8 mmol/g at 3.6 MPa (Rw = 2.55), which is 1.85 times higher than that on a dry

mixture was removed at 323 K in vacuum. Finally, the obtained resol was dissolved in the ethanol solvent. The mesoporous carbon was synthesized by the triconstituent coassembly of resol acting as a carbon source and oligomer silicates from TEOS and triblock copolymer F127 acting as a template.22 F127 was dissolved in the mixture of ethanol and HCl solution at 313 K. The solution was stirred until it was completely dissolved, and then TEOS and resol were added in sequence. After stirring for 2 h, the solution was then put into the dish to evaporate the ethanol for 5 to 8 h at room temperature. Then, it was placed in an air oven at 373 K for 24 h for thermodynamic polymerization. The obtained solid was heated in a nitrogen atmosphere and was kept at 623 K for 3 h and then at 1173 K for another 2 h. The silica template was removed with 5 wt % HF at room temperature. Finally, the mesoporous carbon was washed with deionized water and dried overnight at 393 K in a vacuum oven. 2.3. Characterization. Nitrogen adsorption isotherm curves at 77 K were measured to calculate the mesoporous carbon’s pore size distribution (PSD), specific surface area, and total pore volume. The PSD was evaluated by the Barret− Joyner−Halenda (BJH) theory. The Brunauer−Emmett−Teller (BET) equation was used to calculate the specific surface areas. The total pore volume was estimated from the adsorption capacity of N2 at a pressure of p/p0 = 0.98. 2.4. Sorption Equilibrium Measurement. When incorporating CO2 in the porous material’s preadsorbed water, a CO2 hydrate can form under suitable conditions. The uptake of CO2 on the mesoporous carbon in the presence of water is mainly contributed by gas hydrate and adsorption.13 By convention, the phase equilibrium of CO2 is called sorption. The sorption equilibrium of CO2 was collected with a volumetric experimental apparatus,15,23 and the free volume of the adsorption cell was measured by the expansion of helium.24 To prepare wet carbon, distilled water was added slowly to dry carbon, and it was stirred at the same time. Rw represents the weight ratio of water to dry sample, which varied from 0 to 2.95. The experiment temperature was in the range of 275 to 281 K with an accuracy of 0.1 K, and the highest pressure was about 4.2 MPa. B



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Figure 1. N2 adsorption−desorption isotherms and corresponding pore size distributions of mesoporous carbons with different mass proportions of reagents at 77 K.

MPa.14 After the inflection pressure, a large number of CO2 hydrates formed, indicating that a phase transition took place, and CO2 gas transformed into solid hydrate, resulting in a sharp increase13 in the CO2 sorption capacity in the wet sample. The total pore volume obtained from the adsorption of N2 at 77 K is 1.61 cm3/g. When Rw = 2.05, the volume of water added to the carbon was about 2.05 cm3/g. Some water occupied the macropore, which could not be measured via the N2 adsorption at 77 K. Besides, the smaller the pore size of the sample, the higher the formation pressure of hydrates required25 Therefore, CO2 hydrates respectively form in the

sample (28 mmol/g, 3.6 MPa). The sorption capacities of CO2 for other porous materials are shown in Table 3. The highest sorption capacity (51.8) in this work is higher than that on CMK-3 with preadsorbed water (42 mmol/g, 3.6 MPa). 3.3. Discussion of the Sorption Mechanism. Prior to the inflection pressure, CO2 sorption capacity is mainly affected by the solubility of CO2 in water and adsorption of CO2 on the sample surface, water occupies a big part of the surface in the wet samples, resulting in the sorption capacity of CO2 on the dry mesoporous carbon being much higher than that on preadsorbed water samples as the pressure is lower than 1.75 C



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where ΔH is the enthalpy change, f is the fugacity of carbon dioxide (MPa), T is the temperature (K), R is the gas constant (8.314 J·mol−1·K−1), f = p × Φ, and ln Φ =

∫0

p

(z − 1) dp/p

(2)

where p is the inflection pressure, Φ is the fugacity coefficient, and z is the compressibility factor of carbon dioxide. The plot of ln f versus 1/T is shown in Figure 4. In this work, the enthalpy change calculated from the slope of the line in

Figure 2. CO2 sorption isotherms on the mesoporous carbon sample with different water contents. 1, Rw = 0; 2, Rw = 2.05; 3, Rw = 2.35; 4, Rw = 2.55; 5, Rw = 2.95.

Table 3. Sorption Capacity of CO2 in Porous Materials adsorbent

sorption capacity (mmol/g)

Rw/pressure

12.1 31

2.36/2.5 MPa 1.65/2.2 MPa

15 14

42 51.8

2.35/3.6 MPa 2.55/3.6 MPa

13 this work

bamboo carbon activated carbon (BY1) CMK-3 mesoporous carbon

reference

Figure 4. Clausius−Clapeyron plots to evaluate the enthalpy changes.

Figure 4 was −63.1 kJ/mol, which is higher than the enthalpy changes for CMK-3 (−55.6 kJ/mol)13 and silica gel (−57.4 kJ/ mol)26 and lower than that for the microporous coconut shell activated carbon (−81.9 kJ/mol)14 and silica KIT-6 (−97.1 kJ/ mol).11 These materials had different pore size distributions, surface properties, and water contents, which affected the CO2 hydrate formation. 3.4. Influence of THF Concentration on CO2 Hydrate Formation. The effect of THF concentration on CO2 hydrate formation is shown in Figure 5. The concentration of THF is defined as the molar amount of THF per total molar amount of water and THF, which is expressed as C (mol %).

inner and external spaces of the pores, corresponding to the two inflection pressures (1.75 and 2.50 MPa) in curves 2 to 4.13 For Rw = 2.95, too much water makes it difficult to form CO2 hydrates, so there is little CO2 sorption in curve 5. To calculate the enthalpy change of the phase transition, additional sorption isotherms at different temperatures (277, 279, and 281 K) were measured for Rw = 2.55 as shown in Figure 3. Obviously, the higher hydrate formation pressures

Figure 3. Sorption isotherms of CO2 on a mesoporous carbon sample with Rw = 2.55 at different temperatures: 1, 275 K; 2, 277 K; 3, 279 K; and 4, 281 K.

Figure 5. Sorption isotherms of CO2 of the mesoporous carbon upon adding different THF concentrations at 275 K. C: ■, dry carbon; ○, 0; ▲, 5.1; □, 6.7; and ●,8.2 (mol %).

were required with the higher temperature. There is only one inflection pressure when the temperature is 281 K, and it may be that the first inflection pressure of CO2 hydrate formation increases to overlap with the second. The calculation of the enthalpy change was based on the Clausius−Clapeyron equation ⎡ d ln f ⎤ −ΔH = −R ⎢ ⎥ ⎣ d(1/T ) ⎦n

THF can be used as the thermodynamic promoter to shift the hydrate phase-stability curve to lower pressure.27,28 It is well known that THF can form a structure II hydrate in which THF molecules also fill the large cages while the gas molecules enter the small cages.29 Generally, carbon dioxide forms a structure I hydrate in pure water. The addition of THF can affect the type

(1) D



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of gas hydrate formation, resulting in a change in the hydrate formation pressure.27−29 When the concentration of THF is 5.1 (mol %), there is a significant change in the inflection pressure. The first inflection pressure of curve C = 5.1 (mol %) is about 0.70 MPa, which is remarkably lower than that for curve C = 0 (mol %) (1.75 MPa). Furthermore, it is clear that the first inflection pressures decreased from 0.70 to 0.25 MPa, which decreased upon increasing the concentration of THF from 5.1 (mol %) to 6.7 (mol %). However, the hydrate formation pressure is maintained at 0.25 MPa when a higher concentration of THF (8.2 (mol %)) was added, which indicates that there may be a critical value of the additive to promote hydrate formation. In this work, 6.7 (mol %) THF is closest to the critical value to reduce the hydrate formation pressure. 3.5. Molar Ratio of H2O to CO2. The molar ratio of H2O to CO2 is an important factor that effects the CO2 sorption. In pure water, the CO2 hydrate is structure I with a water to carbon dioxide theoretical molar ratio of 5.75.26 The molar ratios of H2O/CO2 obtained from the CO2 sorption capacities at 275 K and at 3.60 MPa are shown in Figure 6, along with the

Figure 7. Effect of packing density on the CO2 sorption capacity (Rw = 2.05).

MPa for Rw = 2.55, which is 85% higher than that on the dry mesoporous carbon sample and 1.23 times as high as the highest CO 2 sorption previously obtained on ordered mesoporous carbon CMK-3 with preadsorbed water. Because the addition of THF leads to a decrease in the formation pressure of the CO2 hydrate, the energy consumption for CO2 storage can be reduced. Therefore, the as-prepared mesoporous carbon is a promising medium for CO2 storage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-022-27406301. Fax: +86-022-27406301. ORCID

Xiaojing Wang: 0000-0002-1493-3333 Notes

The authors declare no competing financial interest.

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Figure 6. Molar ratios of H2O to CO2. 1, CMK-3; 2, mesoporous carbon.

ACKNOWLEDGMENTS Support from the National Natural Science Foundation of China (nos. 21206108 and 21406004) and the Tianjin Municipal Science and Technology Commission (no. 14JCYBJC21200) is greatly appreciated.

ratio for CMK-3.The molar ratios vary from 2.59 to 2.73 for Rw = 2.05−2.55 in the mesoporous carbon, which is lower than the theoretical ratio of 5.75. This is consistent with previous reports on carbon materials,13−15 and it is caused by the multiple sorption mechanism of CO2 in the mesoporous carbon, which includes CO2 hydrate formation, dissolution in water, and adsorption on the mesoporous carbon.11 Compared to CMK-3 (Figure 6), it is found that the value of H2O/CO2 in mesoporous carbon is lower, indicating that CO2 has a higher utilization rate of water on the mesoporous carbon. 3.6. Effect of Packing Density on CO2 Storage Capacity. CO2 storage capacities of wet mesoporous carbon samples with the different packing densities were measured at 275 K and 3.5 MPa for Rw = 2.05. The results are shown in Figure 7. When the packing density increases to 0.30 g/cm3, the CO2 storage capacity reaches the maximum, 269 V/V(STP). For 0.30−0.45 g/cm3, the packing density increased, and the storage capacity decreased rapidly due to the gas diffusion resistance caused by a high packing density. Therefore, 0.30 g/ cm3 is beneficial to CO2 sorption in this study.

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4. CONCLUSIONS The sorption capacity of CO2 in the mesoporous carbon is remarkably enhanced in the presence of water. The sorption capacity achieved is as high as 51.8 mmol/g at 275 K under 3.6 E



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Triconstituent Coassembly to Mesoporous Carbon and Its Application

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